This invention relates to the fabrication of semiconductor devices via chemical vapor deposition and, in particular, the fabrication of such devices in metalorganic chemical vapor deposition (MO-CVD) with nonplanar layer characteristics by means of masking techniques employed during growth.
It has been established in research and development of semiconductor injection lasers having an active layer and/or cladding layers which are nonplanar and have spatial variation in their thickness exhibit improved properties, such as, low threshold current, linear light output verses current characteristics and stable fundamental transverse mode control. Such nonplanar variations are discussed in U.S. patent application Ser. No. 181,262 filed on Aug. 25, 1980, now U.S. Pat. No. 4,335,461, entitled "Injection Lasers With Lateral Spatial Thickness Variations (LSTV) In The Active Layer" and assigned to the assignee herein and in corporated herein by reference thereto.
To date, such nonplanar lasers have been successfully grown by liquid phase epitaxy (LPE).
Within the past several years, molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MO-CVD) have become important processes in the fabrication of single crystal semiconductor integrated devices, including injection lasers. MBE is a growth process carried out under ultra high vacuum conditions, by the evaporation of the crystal constituents and dopants and beam deposited on substrates. MO-CVD is a gaseous crystal growth technique in which compounds, such as, (CH.sub.3).sub.3 Ga, are caused to react with other gases, such as, AsH.sub.3, and appropriate dopants, in the vapor phase to produce single crystalline or polycrystalline deposits. These two procedures have, to a large extent, replaced the conventional LPE crystal growth techniques, owing to their improved control over (1) layer thickness, (2) crystal composition, (3) layer smoothness, (4) abruptness of interfaces, and (5) uniform doping profiles.
LPE processes permit nonplanar variations in layer contours and thicknesses as desired. For example, LPE growth of channeled substrate lasers produced curved contours and thickness variations in deposited layers on the substrate. However, MBE and MO-CVD processes characteristically do not produce the same type of growth variations. Depending upon deposit rate, flow rate, substrate temperature, etc., the deposited layers or films tend to "match" the contour and shape of the depositing surface. It would be desirable to start with a substrate surface with a curved contour having a curved contour or taper adequate to produce the tapered variations during growth, as taught in the previously mentioned patent application. However, it is not readily easy to fabricate the desired curvature in a substrate prior to growth. It would be simpler to develop the desired contour during growth, as done in the past, and obtain better accuracy and control in the desired contour and thickness variations that MBE and MO-CVD processes would provide.
One way of accomplishing these spatial variations in MBE is by employing a mask having an aperture. The mask is positioned between the elemental sources and the substrate. Only elemental materials propagating through the mask aperture will deposit on the surface of the substrate.
But what about masking in MO-CVD processes?. One would conclude that an apertured mask in MO-CVD will be of little help. MO-CVD involves the flow of gases through a reactor that engage a supported substrate where pyrolyzation of vapor mixtures of elemental compounds in these gases occurs. Turbulance is present in the flow of these gases in the region of the substrate. One would, therefore, postulate that because of the turbulant nature of the gas flow in this region, it would be inept for one to conclude that apertured masking may be a viable way of producing desired layer spatial variations during MO-CVD growth processes. With an apertured mask positioned over the substrate upon which deposition is to occur, the turbulant motion of gases about and in the mask aperture would surely lead to uneven the nonuniform spatial variations in tapered contours and layer or film thicknesses.